An optical fiber is a filament made of dielectric material to guide light. As such, optical fibers can be used as a medium for transmitting information in the form of light energy. In fact, for those interested in transmitting large amounts of information in a short period of time, optical fibers provide many advantages over other communication mediums. For instance, optical fibers made from silica glass provide very small transmission losses, have a greater bandwidth than any other transmission medium known today, and are immune to electromagnetic interference.
In transmitting information over long distances through such optical fibers, however, the optical signal tends to degrade from the effect of chromatic dispersion. That is, the various frequency components of the optical signal have slightly different velocities as the signal travels down the fiber causing each pulse to spread in time.
This problem was partially solved with the advent of the soliton. Solitons are pulses of light that maintain their shape over long distances. Light pulses that travel as solitons can travel much farther along an optical fiber before experiencing dispersive broadening. The soliton holds its shape because the passage of light through the fiber temporarily decreases the speed of light in that part of the fiber. That is, the soliton has the correct amplitude and shape sech(t) such that this nonlinear property can exactly compensate for the temporal spreading due to chromatic dispersion.
The use of optical fibers in such high capacity transmission systems was further enhanced by incorporating wavelength division multiplexing (WDM) therein. WDM is a way of increasing the capacity of an optical fiber by simultaneously transmitting more than one wavelength over the same fiber. Thus, with WDM, one can multiplex signals by transmitting them at different wavelengths over the same fiber, and thus further increase the information throughput over a given time period.
Although WDM soliton transmission systems provide an effective way to increase the capacity of ultra-long distance data transmissions, they may still exhibit severe interchannel interference due to effects of the nonlinear property of the fiber. That is, solitons from different channels can interact through the non-linearity to shift each others optical frequencies and hence shift each others velocities, and to generate new and interfering optical signals through a process known as four wave mixing. Both of these effects can ultimately result in the generation of errors in the digital transmission.
It has been shown, however, that in a fiber having both constant dispersion and negligible loss, solitons of different wavelengths are transparent to each other. This transparency means that each soliton emerges from a mutual collision with wavelength, energy and shape unaltered. In particular, four-wave mixing (FWM) components that make a temporary appearance during the collision are reabsorbed by the solitons, thus maintaining the original shape of the soliton as it travels along the fiber.
It has been asserted that this transparency can be maintained in a system using a chain of lumped amplifiers, as long as the collision length (the distance the solitons travel down the fiber while passing through each other) is two or more times the amplifier spacing. Nevertheless, the analysis making this assertion was focused on the effect of cross-phase modulation between colliding solitons, and the generation of FWM components was assumed to be similar to the lossless case.
In a recent experimental study of soliton WDM transmission at 10 Gbit/s per channel, however, the WDM clearly involved serious penalties. In particular, the distance for error free transmissions were 35, 14, and 7 Mm, respectively, for 1, 2, and 3 channel transmissions. Thus, it was abundantly clear that the assertion of transparency between channels, described above, had overlooked an important effect.
The missing effect is the potential for uncontrolled growth of FWM, due to pseudo phase matching from the periodic intensity fluctuations between amplifiers. Such uncontrolled growth of the FWM imposes penalties on the transmission by two different mechanisms. First, since the energy represented by the FWM fields is not reabsorbed by the solitons, the solitons tend to lose energy with each collision. Since the net energy loss of a given soliton depends on the number of collisions it has suffered, and upon the addition of FWM fields with essentially random phases, it directly creates amplitude jitter. The energy loss leads to timing jitter as well, both through the intimate coupling between amplitude and frequency inherent in filtered systems, and through its tendency to asymmetrize the collision, and hence to induce net velocity shifts. Thus, even in a two channel WDM, there can be serious penalties (see FIG. 1).
Moreover, if the wavelengths of the FWM products coincide with the wavelengths of other WDM channels (possibly only when there exists three or more channels), the run-away FWM becomes an additional source of noise fields to act on those channels. In that way, the well known amplitude and timing jitter effects of spontaneous emission are enhanced. As a result, in WDM soliton transmission systems having lumped amplifiers, constant dispersion fibers fail to compensate for nonlinear effects of the fiber on the traveling signal, and thus have substantial transmission loss.